Currently, many telecommunication networks send and receive information as optical signals. Such networks can provide significantly greater bandwidth than their electrical wire counterparts. One reason for this is that a single optical fiber can carry multiple (e.g., 80 or more) signals on different wavelength channels simultaneously. In practice, such networks are highly dynamic and power in any given channel may fluctuate relative to other channels because of, for example, wavelength drift in network components, channel add/drops, and path reconfigurations. Thus it is often necessary to variably attenuate the intensity of a given optical signal in one or more channels. For example, this may be necessary to minimize channel cross-talk caused by nonlinear interactions between different channels.
In general, in one aspect, the invention features a variable optical attenuator including: a birefringent element positioned to separate an input optical signal into two spatially separated, orthogonally polarized beams; a liquid crystal (LC) modulator positioned to receive the orthogonally polarized beams and selectively alter their polarizations; and a reflective element positioned to reflect the beams back through the LC modulator and the birefringent element, wherein the birefringent element recombines orthogonally polarized components of the reflected beams to produce an output optical signal. The variable optical attenuator may further include a controller coupled to the LC modulator to selectively cause the LC modulator to alter the polarizations of the orthogonally polarized beams. During operation the controller responds to a request to variably attenuate the intensity of the output optical signal relative to the intensity of the input optical signal to one of multiple non-zero attenuation settings.
Embodiments of the variable optical attenuator may further include any of the following features.
The variable optical attenuator may further include an input port positioned to direct the input optical signal into the birefringent element and an output port positioned to receive the output optical signal from the birefringent element.
Moreover, the variable optical attenuator may further include additional input ports each directing an additional input optical signal into the birefringent element and on through the LC modulator and the reflective element, and additional output ports each positioned to receive an additional output optical signal from the birefringent element. Each additional output optical signal corresponds to one of the additional input optical signals after it is reflected back through the LC modulator and the birefringent element by the reflective element. Furthermore, the reflective element may be a right-angle prism, and the additional input ports and the additional output ports can each extend along an axis substantially parallel to a fold axis defined by the right-angle prism.
The LC modulator may further include multiple, independently addressable regions for selectively altering the polarization of an incident beam. For example, the birefringent element may direct a first one of the spatially separated beams to pass through a first one of the addressable regions of the LC modulator and a second one of the spatially separated beams to pass through a second one of the addressable regions of the LC modulator. Futhermore, the reflective element may reflect the first beam to pass back through the first addressable region of the LC modulator and the second beam to pass back through the second addressable region of the LC modulator. Alternatively, the reflective element may reflect the first beam to pass through the second addressable region of the LC modulator and the second beam to pass through the first addressable region of the LC modulator. Furthermore, the reflective element may reflect the first beam to pass through a third addressable region of the LC modulator and the second beam to pass through a fourth addressable region of the LC modulator.
During operation the controller may drive the multiple regions of the LC modulator to cause an intensity ratio of the orthogonally polarized components of the output signal to substantially equal an intensity ratio of the orthogonally polarized beams derived from the input signal.
The LC modulator may include a LC layer sandwiched between a first substrate supporting a ground electrode and a second substrate supporting multiple electrodes corresponding to the multiple, independently addressable regions. It may further include a fixed retarder layer in series with the LC layer. The LC layer may include nematic LCs aligned with respect to an alignment axis in the plane of the LC layer. For example, the alignment axis may be at angle of about 45xc2x0 to an axis defined by the spatial separation of the orthogonally polarized beams derived from the input beam. Alternatively, the LC layer may include twisted nematic LCs.
In some embodiments, the variable optical attenuator further includes a dichroic polarizer positioned between the LC modulator and the reflective element. For example, the dichroic polarizer may be positioned to absorb a selected polarization component of the beams for a first pass from the LC modulator to the reflective element and for a second pass from the reflective element back to the LC modulator. The dichroic polarizer may have a dichroic axis aligned either parallel or orthogonal to an axis defined by the spatial separation of the orthogonally polarized beams derived from the input beam.
The reflective element may be, for example, a mirror oriented to receive the beams at a non-normal angle, a corner cube retroreflector, or a right-angle prism. In the latter case, the variable optical attenuator may further include a retarder element positioned between the LC modulator and the right-angle prism. The retarder element may be configured to offset polarization-dependent phase changes to the beams caused by non-normal reflections from the right-angle prism. The retarder element may be further configured to compensate for a geometric inversion caused by the right-angle prism.
The variable optical attenuator may further include a LC monitor coupled to the LC modulator and the controller. For example, the LC monitor may include a reference light source providing a polarized source beam. The reference light source may include an LED or a laser diode, in series with a film polarizer. During operation, the reference light source directs the polarized reference beam through an active region of the LC modulator to produce a signal beam. The LC monitor further includes a polarizer positioned to receive the signal beam and produce a polarized signal beam, and a detector for monitoring the intensity of the polarized signal beam.
The LC modulator may provide a tunable retardance spanning a range of less than 450 nm for a single pass.
In general, in another aspect, the invention features a variable optical attenuator including: a birefringent element positioned to separate an input optical signal into two spatially separated, orthogonally polarized beams; a LC modulator positioned to receive the orthogonally polarized beams and selectively alter their polarizations; a reflective element positioned to reflect the beams back through the LC modulator and the birefringent element, wherein the birefringent element recombines orthogonally polarized components of the reflected beams to produce an output optical signal; and a dichroic polarizer between the LC modulator and the reflective element, wherein the polarizer is positioned to contact the beams during at least one of a first pass from the LC modulator to the reflective element and a second pass from the reflective element back to the LC modulator.
Embodiments of the variable optical attenuator may further include any of the following features. The polarizer may be positioned to contact the beams during both passes. The dichroic polarizer may have a dichroic axis aligned either parallel or orthogonal to an axis defined by the spatial separation of the orthogonally polarized beams derived from the input beam. The LC modulator may include multiple, independently addressable regions for selectively altering the polarization of an incident beam.
In general, in another aspect, the invention features a variable optical attenuator including: a birefringent element positioned to separate an input optical signal into two spatially separated, orthogonally polarized beams; a LC modulator positioned to receive the orthogonally polarized beams and selectively alter their polarizations; and a right-angle prism positioned to reflect the beams back through the LC modulator and the birefringent element, wherein the birefringent element recombines orthogonally polarized components of the reflected beams to produce an output optical signal.
Embodiments of the variable optical attenuator may further include any of the following features.
The variable optical attenuator may further include a retarder element positioned between the LC modulator and the right-angle prism, wherein the retarder element is configured to offset polarization-dependent phase changes to the beams caused by non-normal reflections from the right-angle prism. The retarder element may be further configured to compensate for a geometric inversion caused by the right-angle prism.
The LC modulator may include multiple, independently addressable regions for selectively altering the polarization of an incident beam.
The variable optical attenuator may further include a dichroic polarizer between the LC modulator and the right-angle prism, wherein the polarizer is positioned to contact the beams during at least one of a first pass from the LC modulator to the right-angle prism and a second pass from the right-angle prism back to the LC modulator.
The variable optical attenuator may further include an input fiber array positioned to launch the first mentioned input optical signal and additional input optical signals into the birefringent element, and an output fiber array positioned to receive the first mentioned output optical signal and additional output optical signals from the birefringent element. For example, the input fiber array and the output fiber array may each extend along axes substantially parallel to a fold axis defined by the right-angle prism. Furthermore, the transverse position and orientation of the right-angle prism can be selected to optimize the coupling of each output beam to a corresponding fiber of the output fiber array. Moreover, to accommodate the multiple input beams, the LC modulator may include multiple, independently addressable regions extending along multiple directions for selectively altering the polarization of an incident beam.
In general, in another aspect, the invention features a method for variably attenuating an input optical signal to one of multiple non-zero attenuation settings. The method includes: separating the input optical signal into two spatially separated, orthogonally polarized beams by directing it through a birefringent element; selectively altering the polarizations of the orthogonally polarized beams based on a desired attenuation setting by directing the orthogonally polarized beams through a LC modulator; and reflecting the beams back through the LC modulator and the birefringent element, wherein the birefringent element recombines orthogonally polarized components of the reflected beams to produce an output optical signal. Embodiments of the method may further include features corresponding to any of the variable optical attenuator features described above.
Embodiments of the invention may include any of the following advantages.
The folded optical arrangement provides a compact structure that obviates the need for a second birefringent element. Furthermore, because the same birefringent element is used to separate and subsequently recombine orthogonal polarization components, the thickness of the element does not need to be tightly controlled. The folded arrangement also provides a double pass through the LC modulator, thereby providing the performance of a two-stage LC device for the cost of one.
Similarly, in those embodiments employing the dichroic polarizer, the folded arrangement provides a double pass through the dichroic polarizer, thereby doubling the extinction ratio of the polarizer. Moreover, the combination of the folded arrangement and the dichroic polarizer provides two stages of attenuation, thereby doubling the attenuation range of the device as a whole.
Furthermore, embodiments that include the right-angle prism can include a compensating retarder that offsets polarization-dependent phase shifts caused by the two reflections in the prism.
The right-angle prism also provides a configuration that easily accommodates multiple input beams. Moreover, translation and/or rotation of the right angle prism provide degrees of freedom for aligning the resulting output beams with a corresponding fiber array. Embodiments that involve multiple input beams can be used for spectral band equalization in wavelength division multiplexing (WDM) optical systems.
Finally, embodiments that include the retardance monitor enable in-situ compensation of temperature sensitivities and aging.
Other features, objects, and advantages of the invention will be apparent from the following detailed description.